Stainless steel bowsprit

[mocruising]

I needed a stainless steel bowsprit to fly a furling genakker on a 48 foot yacht (Hallberg Rassy 46). I had a pattern from a similar boat in the marina and a copy (?) was made up in Lagos Portugal, by a ss fabricator. We did not know the wall thickness of the pipe of the sample that we had but it was 50mm tube (OD). The fabricator suggested 3mm tube wall thickness and went ahead. The bowsprit had a shoe fitting on the deck and then was pinned through the spare stem head roller and extended forward about 50 cm. To cut a long story short on the first use in 10 kts of wind the pipe buckled. So now I have to make a new one but I thought I would be a bit more scientific about it. Any suggestions. Is there a formula for calculating loads on the three corners of a sail given the sail area and wind speed.

Any help would be appreciated.

 

[PaulJ]

I don't have any formulae I'm afraid but 50mm tube with a 3mm wall is clearly nowhere near strong enough. My 39ft steel boat has a 700mm bowsprit and on the plan it is specified as 100mm sq.section with 5mm wall thickness. There is a bobstay bracing it underneath and nothing either side....... and that only has a high cut Yankee hanging on it!

In the event we couldn't get any 100mm sq. section but the fabricator had some 80mmx120mm with 5mm walls and that was approved by the designer, Alan Pape. Perhaps you should contact a designer, I am sure the charge would not be high for one calculation......?

 

[Anchorite]

Here are a few rough figures of force on sails: Force 3 - 3 kg/m2, Force 4 - 5 kg/m2, Force 5 - 10 kg/m2. Your (skinny) sprit buckled in a F3...was it a construction tube or a decoration tube: the latter are usually round figures, construction tube isn't. Same for wall thickness. Buckling rather than banana-ing sounds as if it might be a thin wall (decoration, okay for ladders) or maybe a design fault (sharp fulcrum). Had you toggles top and bottom? Forces are distributed to the three points more or less evenly on a sail set flying: with a hanked or in-the-groove luff a higher percentage is put onto the head and tack. (Aside: I think Paul's bowsprit was designed to win in close combat situations! Trop fort n'a jamais manqué!

 

[PaulJ]

"Aside: I think Paul's bowsprit was designed to win in close combat situations!"..... Alan Pape is/was not known for "designing light", that is one reason why I chose one of his designs! ;o)

Paul

 

[roly_voya]

Just seen this and you have probably already solved the problem bur if not I suggest you get hold of Brian Toss's book o rigging which gives all the formula to caculating rigging load then Ian nicholson's Boat Data Book which gives tables of compression loads for beams, put it all into a calculator and problem solved